This invention relates to a machine for drilling a hole in a workpiece, inserting a rivet in the hole, and upsetting the rivet, all without moving the lateral position of the machine or the workpiece, and to an assembly for supporting and positioning a plurality of said machines for working simultaneously on the workpiece.
Riveting machines are well known and in wide use throughout the aerospace industry as well as other industries. Rivets provide the best known technique for fastening an aerodynamic skin to a frame to provide a strong aerodynamically smooth surface. Rivets are also used in the interior structure of the aircraft since they are the lightest and least expensive method of fastening structural components together. However, inserting and upsetting rivets is a labor intensive process and for the most part is done manually, one rivet at a time. The work is extremely tedious and yet at the same time requires a highly skilled operator to produce quality rivets consistently, and highly skilled quality control inspectors to insure that all of the rivets meet the specifications of flushness, interference and button formation. These personnel costs substantially increase the cost of a riveting operation and tend to offset the inherent low cost of the rivet itself. The process appears susceptible to automation, and attempts have been made for many years to obtain the benefits of increased capacity and quality while reducing cost and rework, but attempts to develop automated riveting installations have been hampered by a multitude of practical problems which interfere with the smooth operation of an automated system, resulting in a requirement for continual manual intervention by skilled operators.
A riveting operation is often so noisy that, in a factory where large scale riveting is being done, hearing protection is mandatory. Much effort has gone into quieting the riveting process to protect employees from hearing damage, but the workplace remains noisy and hearing protection remains mandatory. Hydraulic rivet squeezers are quiet, but they require massive mounting structures to withstand the reaction forces exerted by the hydraulic squeezer. The cost and size of these mounting structures prohibit their use in many applications.
The force to upset a rivet is typically on the order of five to fifty thousand pounds. This force must be exerted from both sides of the rivet, either by a reaction force through a large C-frame, a reaction mass, or an active force generator on both sides of the rivet. Hand-held pneumatic riveters are one simple solution but require two skilled operators, are not always repeatable, are noisy, and have been linked with carpel tunnel syndrome. The ideal riveter would be an electromagnetic, one or two blow riveter acting virtually simultaneously on both sides of the rivet.
A typical automated drill/rivet machine has a drill which will drill a hole into the workpiece and then shift to one side so that the rivet may be inserted. The rivet insertion mechanism then shifts to one side so the rivet may be upset by a rivet die acted on by a driver. The shifting of the drilling and insertion mechanisms is time consuming and requires extremely precise positioning mechanisms in order to maintain the necessary alignment of the drill, the rivet inserter, and the rivet die and driver. A preferable technique would be to perform all three functions from the same lateral position of the device so that no indexing of the major components is required. Some lateral movement will still be necessary to feed the rivet into the hole and position the die over the rivet so that the rivet can be upset. The moving structure for this lateral shifting should be made as light as possible so that it can be indexed from position to position quickly and with great precision.
The drill chips that are produced by the drilling operation are potentially troublesome because they can interfere with movement of the die shuttle mechanism and, if they get between the workpiece and the pressure foot which clamps the workpiece in place, the pressure foot will embed the chips into the workpiece and mar it, and could affect drill or countersink depth. The conventional technique for disposing of chips is to provide a nozzle to blow the chips away from the work site or a suction hose to suck the chips off of the workpiece after they have fallen onto it. These techniques reduce the problem by removing the majority of the drill chips, however there are always some chips that are not removed and these can cause problems and must be periodically removed from the work site by an operator with an air hose. A drill rivet machine would be much more reliable and produce a product with much fewer defects if a reliable chip removing system were available to remove all of the drill chips before they can even come into contact with the workpiece so that the workpiece and the work site are kept clean and free of drill chips.
The drill in the drilling operation must be periodically lubricated to preserve the life of the drill, to maintain hole quality, and to speed the drilling operation. However, in many applications the lubricant is considered a contaminant to the workpiece because it adversely affects the coatings on the workpiece or because of subsequent operations which must be free of lubricant on the surface. The best available technique for lubricating the drill without contaminating the workpiece is a lubricant spray mist system which blows a mist onto the drill and lubricates only the drill and not the workpiece. However, the mist tends to settle to some extent on the workpiece and therefore the mist lubrication system is not one hundred percent effective in maintaining the cleanliness of the workpiece. A lubrication system for a high speed drill rivet device should lubricate the drill before every drilling operation and should reliably protect the workpiece from contamination by the lubricant.
A drill rivet device must accomplish several operations and do so quickly and precisely. It would be desirable during drilling, rivet inserting, and rivet upsetting that the work- piece be clamped securely in a single unmoving position so that the axis of the machine remains aligned with the position at which the rivet is to be placed, despite reactive flexing of the holding fixture. The drill and the rivet upsetting device should be mounted for precise axial movement while the work- piece is held in a clamped position, but then must be mounted so that it can be retracted and the workpiece unclamped after the rivet is placed to enable the workpiece or the machine to be moved to the next position where the next rivet is to be placed. Ideally, all of these functions should be accomplished in a single small, light weight mechanism that would make it possible for the drill rivet machine to occupy only a small volume so that it does not interfere with other adjacent mechanisms.
Within the frame which guides the drill and the rivet upsetting mechanism, the axial movement of the drill and the riveter must be guided, cushioned, damped and positioned so that it accomplishes the functions for which it is intended at high speed and low impact. In an electromagnetic riveter, the impact created when the coil is energized should be absorbed in a recoil cushioning system which enables the machine to take advantage of the possibilities for low reaction force in the electromagnetic riveter. However, the translations involved must be held to a minimum to insure that no wasted motion occurs so the machine can operate at its highest possible cycle rate.
One of the factors that has delayed the development of automated riveting operations in the past has been the large mass of the equipment that must be moved and precisely indexed to the location at which a rivet is to be placed. Improvements have been made but the lateral movement of mechanisms remains a problem and causes the machine cycle time to be lower than it potentially could be. Exacerbating the problem is the multitude of functions which must be performed and the substantial precision with which these functions must occur in precise alignment over the rivet placement location. This precision often degrades as the machine ages and the guideways for the shuttle mechanisms wear.
One of the functions in an automatic riveting machine that must operate reliably is the rivet feeding and insertion device. In some applications, the rivet is normally fed to the machine from a rivet blow feeder through a rivet feed tube, in which the rivet can attain substantial velocity in order to sustain fast machine cycle times. If the rivet were to impact the workpiece at its maximum velocity in the feed tube, it could damage the work- piece in the marginal regions around the hole in which the rivet is to be inserted and could also damage the rivet itself. These dents and nicks in the rivet and the workpiece regions around the hole can influence the sealing of the rivet in the hole and also potentially prevent the rivet from entering the hole at all. Thus it is necessary to insure in an automated riveting operation that the rivet approaches the workpiece at a velocity that is fast enough to carry the rivet into the hole but not fast enough to damage the rivet or the workpiece
Rivet attitude as it approaches the hole should be controlled so that it enters the hole without jamming against the edges of the hole or otherwise jamming in the feeding operation. In conventional rivet feeding systems, the rivet is held in a rivet gripper and is inserted in the hole. This system works well most of the time, although occasionally the rivet gets cocked in the hole and the rivet dies smash it in the cocked position, creating a difficult repair job. However, it does require a finite time for the rivet gripper to place the rivet in the hole, and is just-one more thing to wear out of tolerance. A preferable system would have no moving parts for rivet insertion, and would be virtually instantaneous in the insertion of the rivet in the hole, for minimum machine cycle time.
The rivet, once seated in the hole, must remain in position until it is upset. Some rivet operations are performed in an upside-down orientation (that is, with the headed end of the rivet facing downward) and if the rivet is not held in position it could fall out before it is upset, or it could slide out of position and jam the feed mechanism or the rivet die shuttle. The holding system must function from the time the rivet is inserted until the time the rivet is upset so that the rivet is positively held in the correct position at all times.
The rivet feeding tube usually enters the machine from the side because the rivet die driver and drill are in an axial position and could interfere with the path of the rivet coming axially into the hole. Thus the rivet enters laterally into the machine and then follows a curved path to straightened its line of travel and align it with the hole, which is on the machine axis. The change in direction of the rivet is a tricky operation because several different types and sizes of rivets may be fed in a automatic riveting operation and although the rivet can be softer material such as aluminum it still can cause considerable wear in a curved feed path. Moreover, the tighter the bend that the rivet must execute in going from the lateral approach to the axial path of travel to enter the rivet hole, the more likely it is that the rivet will jam in the bend.
When the rivet exits the bend, it is in an unstable condition and must be straightened and stabilized so that when the rivet leaves the end of the feed structure and moves into the hole, it will travel in a stable condition and does not become canted, so that it enters the hole straight without becoming jammed diagonally in the hole.
A rivet is precisely sized for the thickness of the workpiece which it is to hold together and the stress which it is to carry. The impact energy of the rivet driver is designed to completely form the button end on the rivet and cause the desired degree of interference between the rivet shank and the hole, and between the rivet head and the surface of the workpiece, in the case of Briles rivets. Any substantial deviation from the design parameters will result in an improperly formed rivet or a damaged workpiece. For example, a rivet which is too long will have more material to be strained than the driver-has energy to strain and therefore the rivet will be incompletely upset, resulting in a insufficiently formed button on the end of the rivet and inadequate degree of interference between the shank and the hole, and between the head and the surface of the workpiece. Similarly, a rivet which is too short will have an insufficient amount of rivet material to absorb the driver impact and therefore the rivet tail will be flattened and the energy in the driver will have to be absorbed by the workpiece, resulting in a deformed or a "dimpled" workpiece, or a rivet head pushed off of its countersink. These defects require rework which is expensive and slows the production output of the plant. It would be desirable to design a system for measuring the length of the rivet before it is upset to insure that these defects do not occur.
In the event that a rivet is detected that is too long or too short there should be some method for removing the missized rivet from the hole and disposing of it so that a properly sized rivet can be inserted. This operation should be done quickly and reliably so that the efficiency to be gained by an automatic riveting operation is not lost by these recovery operations. Ideally, the rivet measuring technique and the rivet removing and disposing technique can be incorporated in mechanisms which are part of the automatic drill/rivet device without adding undue complication or increased cost to the device.
An electromagnetic actuator for an electromagnetic riveter has a high amperage coil which develops considerable heat from electrical resistance over a period of use. The heat raises the electrical resistance of the coil and therefore the voltage must be raised, thereby reducing the efficiency of the operation. More importantly however the temperature of the coil must be held within certain limits to prevent the electrical insulation and other materials from reaching breakdown temperatures. The conventional methods for coil cooling employ an airflow through and around the coil to extract the heat and carry it away in the air stream. However, the direct cooling of the coil in this manner requires that the air be dry and of high purity. Water vapor in the air striking the hot coil can cause corrosion of the coil, and impurities in the air can collect in and around the coil to cause bridging of the coil windings. The air treatment system is reliable and available, but it is expensive, requires periodic maintenance and is bulky. An improved coil cooling method for an electromagnetic actuator, that does not require the expensive drying and filtering equipment, would be a great benefit to the operators of electromagnetic actuators in high power equipment such as an electromagnetic riveting machine.
The rivet forming technique for slug rivets includes striking the rivet on both ends simultaneously so that the rivet shank is deformed to provide shank/hole interference, and the rivet head is deformed to provide a properly formed button at each end of the rivet. This provides the optimum rivet strength. Headed rivets present a slightly more complicated problem. Headed rivets already have a head formed at one end of the rivet, normally tapered so that it fits into a countersunk hole in the surface of the workpiece. In order to provide the proper holding effect, the rivet must be installed in such a way as to insure that the head will engage the work- piece countersink with sufficient interference or pressure when the riveting operation is completed. With the old fashioned pneumatic riveting machines, which deliver multiple blows against the rivet, it was possible to determine by simple inspection when the rivet had been sufficiently deformed because the incremental amount of deformation created in the rivet by each blow is so small that a skilled operator could inspect the rivet and see that the operation was complete for that rivet. Additionally, he could tell by the sound and feel of the machine when the rivet was deformed sufficiently to create an acceptable rivet. However, the situation for electromagnetic riveters is more complicated because the entire rivet upset operation must be accomplished in one or two blows, so the blows must be timed and powered to accomplish the entire rivet upset operation completely. The prior art has always delivered the identical blow to both sides of the rivet because of the apparently logical assumption that identical blows will prevent asymmetric effects on the rivet and prevent damage to the workpiece. However, we have discovered that the functions to be performed at each end of the rivet are considerably different from each other and therefore the blow to be delivered to the rivet should be tailored to the function which it is to perform. Since these functions are different for different kinds of rivets, the parameters of the blow to be delivered need to be adjusted in each case.
A riveting machine which drills holes in a workpiece, inserts a rivet and upsets the rivet, all automatically and at high speed, should be under precise automatic control and provide feedback to the controller that every operation has been completed before the next operation can be started so that the danger of jamming or damaging the machine by clashing of subcomponents is minimized. This implies that the machine be provided with sensors which indicate when a operation has been completed and with automated control rules which cause the machine to automatically halt when a failure has occurred to prevent massive damage to the machine or workpiece. The suite of sensors should be as simple and reliable as possible to minimize the cost and maintenance requirements, and should provide all the sensing operations necessary to generate the feedback data needed for the machine to function reliably. Finally, when a failure occurs, the sensor and control system should provide information to the operator or the maintenance personnel as to the cause of the failure was so that corrective action can be taken to identify the problem, clear the blockage or make any repairs that are necessary.
The bearing support at the lower end of the drill and the EMR driver in a concentric arrangement is a desirable function because without it the drill tends to wander and produce a misplaced or misshapen hole, and because the driver should produce its force pulse at exactly the right location to minimize the chances of damaging the machine or the workpiece. The prior-art techniques for accomplishing these functions include special drills with bearing surfaces, but that is an expensive approach that requires the stocking of special parts whose unavailability would make the machine unusable. Moreover, they also typically require the stacking of tolerances and clearances which are additive and can result in excessive play at the free end of the drill or the driver.
The coil of the EMR is electrically insulated from, but in mechanical contact with a transducer of high electrical conductivity. The convention arrangement is to put the coil in contact with the transducer and the transducer in contact with the driver, so that when a burst of current is sent through the coil it will create a rapidly increasing magnetic field which will induce a current in the transducer, which in turn generates an opposing magnetic field and creates a strong repulsive force between the transducer and the coil. The transducer forces the driver against the rivet die to upset the rivet. Arrangements of this type have worked well for numerous years but suffer from the placement of the heavy cable that carries the large amperage current to the coil. Since the coil recoils away from the transducer and the driver along with the recoil mass, a substantial translation of the coil occurs, which means that cable must be capable of withstanding this severe whipping motion. The cable thus must be reinforced and strengthened so that it does not suffer damage from fatigue effects of these continual sharp translations. It would be desirable if the coil could be mounted in such a manner that it experiences a very small translation so that the extra reinforcement in strengthening of the cable would be unnecessary.
Maintenance of an electromagnetic riveter, particularly when it is incorporated in a concentric drill/rivet machine, can require substantial nonproductive time during which the machine is being disassembled, serviced, and then reassembled. It would be of great practical value if the disassembly time could be reduced and the machine designed in modules that could be replaced with new modules when worn, so that the nonproductive time could be reduced or eliminated altogether.